Conventional thin film solar cell arrays which utilize amorphous silicon (hereinafter referred to as "a-Si") have a plurality of unit cells connected in series for the purpose of increasing the output voltage. Generally, each unit cell is composed of: a transparent electrode formed of ITO (indium-tin oxide) or SnO.sub.2 and arranged on a transparent insulating substrate, such as a glass substrate; an a-Si layer, having a p-i-n junction structure, and being composed of an approximately 200 .ANG. thick p-type layer formed by glow-discharge decomposition of a gas mixture of silane, hydrocarbon such as acetylene, and diborane, an approximately 0.5 .mu.m thick non-doped layer, formed by glow-discharge decomposition of silane gas, and, an approximately 500 .ANG. thick n-type layer, formed by glow-discharge decomposition of a gas mixture of silane and phosphine; and, a rear electrode formed of an approximately 1 .mu.m thick metal thin film.
The conventional series connection is established as follows. First, a transparent electrode film, an a-Si film and a rear electrode film are successively formed on one and the same transparent insulating substrate. The whole surface of the substrate is covered with the respective films. After each respective layer deposition, the respective film is patterned to separate unit cells and to form connection portions between adjacent cells. More specifically, after the transparent electrode film is formed by electron beam evaporation or sputtering, the patterning of the transparent electrodes is made by a printing method or photolithography in which a resist pattern is formed by exposure using a photomask for etching treatment. The patterning of the a-Si film is made by a photolithography or laser-scribing method. The metal rear electrode film is formed by electron beam evaporation or sputtering method and is patterned by photolithography.
Conventionally, an expensive apparatus for evaporation or sputtering is used. Additionally, photolithographic techniques are used to form the rear electrodes of a thin metal film. Accordingly, it is difficult to reduce the number of manufacturing steps. Typically, in order to reduce the manufacturing costs, printing of the rear electrodes has been suggested as a means to make it possible to perform simultaneously the film forming process and the patterning process.
In the case of a crystalline Si solar cell or a polycrystalline Si solar cell, the electrodes can be formed by coating the Si layer with a paste. The paste may be prepared by mixing Ag particles in epoxy resin and baking the paste at 600.degree.-700.degree. C. so that electrodes which come into electrical contact with the Si layer are formed. FIG. 2, curve 10, demonstrates the result where this technique is applied to an a-Si solar cell, using an illuminance of 200 lx. The fill factor is not larger than 0.4. FIG. 2, curve 20, illustrates the characteristic of a solar cell for an incident energy of 100 mW/cm.sup.2. Heating of the a-Si solar cell to 200.degree. C. or more is impossible. Therefore, sintering, due to vaporizing of a high molecular weight resin, cannot be performed and a sufficiently low contact resistance cannot be secured. Finally, conventional printed electrodes suffer from poor resistance to humidity.